The present invention relates to a hydraulic valve lash adjuster for taking up clearance in a valve train in an internal combustion engine, and more particularly to a lash adjuster in a valve lifting mechanism for an overhead-camshaft engine.
In a valve lifting device of an overhead-camshaft (OHC) engine, a hydraulic valve lash adjuster is used for taking up the clearance in the valve train. One type of such hydraulic adjusters comprises a cylindrical body secured in a wall of an engine case, a hollow plunger slidably engaged in the cylindrical body, passages for supplying lubricating oil in the hollow plunger passing through holes formed in the cylindrical body, a pressure chamber disposed behind the plunger, a ball check valve provided in the pressure chamber to close a valve port in the rear wall of the plunger to maintain pressure oil therein, and a spring disposed in the pressure chamber to urge the plunger outwardly. The tip end of the plunger engages with one end of a rocker arm, a cam engages with the rocker arm at an intermediate position of the arm, and the other end of the rocker arm engages with the top end of the valve stem.
If such a hydraulic adjuster is employed in the horizontally opposed cylinder type engine, the adjuster must be horizontally disposed in the wall of the engine case. However, the hydraulic adjuster does not effect the lifting of the valve in the horizontal disposition because of leakage of oil during an unoperated condition of the engine. More particularly, the lubricating oil in the plunger drains through a clearance between the plunger and the cylindrical body and in place of oil air enters the plunger, when the engine is stopped. In addition, if the engine is stopped at an open condition of the valve, where the plunger is forced into the cylindrical body and the pressure chamber contracts to the smallest capacity, the oil in the pressure chamber discharges through the clearance between the plunger and the cylindrical body. Therefore, when the engine is started under such a condition, the plunger is projected by the plunger spring disposed in the pressure chamber. As a result, the pressure chamber expands to induce the air in the plunger therein. Accordingly, the plunger cannot be held to a predetermined position due to the inducted air, so that the clearance in the valve train is not taken up. This means that the valve is not lifted to a predetermined position and the cam strikes the rocker arm, causing noise.